1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more specifically to the so-called air intake side secondary air supply system for an internal combustion engine.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air-fuel ratio of the engine is controlled around a stoichiometric value (14.7:1 for example) by a feedback control in accordance with the composition of the exhaust gas and operating conditions of the engine since an optimum operation of the three-way catalytic converter is enabled at the stoichiometric air-fuel ratio. The air intake side secondary air supply system for the feedback control is constructed such that an air intake side secondary air supply passage leading to downstream side of the throttle valve is provided and the air fuel ratio control is performed by varying the amount of the secondary air flowing through the air intake side secondary air supply passage.
As an example of the air intake side secondary air supply system, there is a system performing a pneumatic type PI (proportional and integral) control operation which is described in Japanese Patent Application No. 57-2127548 assigned to the same assignee of the present application. In this system, there are provided a pair of air intake side secondary air supply passages, both communicated to the downstream side of the throttle valve. The air-fuel ratio is detected in terms of a exhaused gas oxygen concentration and an air-fuel ratio signal is produced. The communication through one of the air intake side secondary air supply passages is controlled by means of an open/close valve operated in accordance with the air-fuel ratio signal. The other one of the air intake side secondary air supply passage is provided with an air control valve whose opening degree is controlled by the magnitude of the pressure applied to a pressure chamber thereof. The pressure chamber of the air control valve is supplied with one of first and second control pressure for respectively opening and closing the air control valve so that the sectional area of the other one of the air intake side secondary air supply passages is gradually increased or decreased.
With this type of arrangement, the air-fuel ratio is pneumatically controlled throughout a very wide control range.
However, in this type of pneumatic system, it was necessary to provide further devices such as an additional air control valve in the first one of the air intake side secondary air supply passages in order to control the amount of the secondary air flowing therethrough in accordance with the operating conditions of the engine. In addition, it was necessary to provide a three-way solenoid valve in order to introduce one of the first and second control pressure into the pressure chamber of the air control valve disposed in the second one of the air intake side secondary air supply passages. Thus, the problem of this type of pneumatic system was that a number of parts are required and the construction becomes complicated.
Thus, it is desired to provide a relatively low cost system of pneumatic type even through this type of systems do not cost higher than conventional air intake side secondary air supply systems of the type of electrically operated PI operation in which the control operations are performed by pulse motors.
An object of the present invention is therefore to provide an air intake side secondary air supply system of the type of pneumatic control which is relatively simple and costs less, while capable of the air fuel ratio control through a wide range without causing the deterioration of the driveability of the engine.
According to the present invention, the air intake side secondary air supply system includes first and second secondary air supply passages, both communicated to the downstream side of the throttle valve. The first secondary air supply passage is provided with an air control valve for controlling the amount of air flow according to the magnitude of the pressure supplied to a pressure chamber thereof and the second secondary air supply passage is provided with an open/close valve operated by an air-fuel ratio signal produced in accordance with an oxygen gas concentration. The invention features a delay means provided in the second secondary air supply passage, on the upstream side of the open/close valve and a pressure supply passage which provide a communication between the pressure chamber of the air control valve and a part of the second secondary air supply passage between the open/close valve and the delay device whereby the vacuum of the downstream side of the throttle valve is supplied to the pressure chamber of the air control valve through the second secondary air supply passage and the pressure supply passage when the open/close valve is open.
Further scope and applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating a preferred embodiment of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.